Rational Design of Heterojunction Interface for Cu2ZnSn(S,Se)4 Solar Cells to Exceed 12% Efficiency

The photovoltaic performance of the kesterite Cu 2 ZnSn(S,Se) 4 solar cells is still far below its predecessor CuInGaSe 2 . One major reason is its severe interface nonradiative recombination at the mismatched Cu 2 ZnSn(S,Se) 4 /CdS heterojunction interface, leading to a large open‐circuit voltage loss. Herein, a distinctive indium‐incorporation (DI) strategy is developed to deposit an In 1− x Cd x S buffer layer for optimizing the heterojunction interface. The results reveal that adopting this DI method can effectively inhibit the formation of an undesirable secondary phase, and indium can be more easily doped into the bulk lattice of CdS to form additional beneficial shallow donor In Cd defects, which significantly improve the electrical properties of the CdS layer and the quality of the heterojunction interface. Besides, the energy band alignment is adjusted to facilitate the extraction and transfer of interfacial charges, and thus reducing the nonradiative charge recombination at the front Cu 2 ZnSn(S,Se) 4 /CdS heterojunction interface. Consequently, the combination of the above enhances the power conversion efficiency from 10.2% to 12.4%, one of the highest for this type of cells, which corresponds to an open‐circuit voltage deficit ( V oc,def ) reduction to as low as 0.54 V. The strategy provides a rational design for optimizing the heterojunction interface of Cu 2 ZnSn(S,Se) 4 solar cells to reduce voltage loss and achieve high efficiency.